Possessives and Attributives
Q. This question has probably been asked before, but at work we are updating the human resources manual and nobody seems to know the answer. Is the apostrophe necessary in “two weeks’ notice” and “three days’ sick leave”? We will really appreciate your advice.
A. Yes, it has been asked before! Luckily for you, we are the soul of patience. The apostrophe is necessary, since those phrases express a type of possessive. Please see CMOS 7.24 (Possessive with genitive).